This invention relates to a control system for controlling boiler inputs in a once-through boiler so as to maintain the throttle pressure of the boiler at a desired value while maintaining the required steam flow to a load, such as a turbine-generator.
One of the most difficult problems that face the designer of control systems for fossil fuel boilers of the once-through type is the development of a fuel measurement that is adequate for meaningful control of the firing rate.
It used to be that utilities could depend on a single source of fuel supply for long periods of time. The absence of such dependable supplies along with the high cost of energy as well as increasingly stringent emission standards have forced utilities to obtain their fuel supplies from various sources depending on price and availability. Thus a constant heating value for any given fuel can no longer be assumed. This problem is further complicated by the industry shift back to coal firing which has always been subject to significant heating value variations. The problem of variable heating values exists for all fuels; however, the problem is particularly evident in coal fired systems.
In addition to the problem of changing heating values in coal delivery systems, these systems present other problems in that the fuel rate measurement typically doesn't reflect the actual coal flow to the furnace due to two main considerations: mill storage capacity and mill response capability. Mill coal flow is typically controlled by adjusting feeder speed, using a tachometer output to measure feeder speed and hence fuel input. All mills have significant capacity so that fuel flow into the mill does not necessarily represent the actual fuel flow going to the furnace of the boiler, let alone the BTU input to the boiler. Typical time constants for a mill may be in the range of 100 to 160 seconds before a change of mill input will be seen as a significant output to the furnace. These fuel delivery system dynamics may advantageously be taken care of with a "Mill Model" using feeder speed as its input. This model would typically have a dead time and a first order lag.
The difficulties created by trying to match fuel input energy required against actual energy delivered, particularly for those fuels with different heating values precipitated efforts of a coworker of mine to develop a method of balancing energy supply for a boiler with energy demand, regardless of the fuel service. A heat release calculation using turbine first stage pressure and rate of change of drum pressure was an integral part of a successful solution for matching energy supply to energy demand at the fuel controller for drum type boilers and resulted in U.S. Pat. No. 4,213,304, issued on July 22, 1980 to Richard H. Morse. That patent is hereby incorporated by reference into this application.
It is an object of this invention to provide a method and means for controlling a boiler of the once-through type in a manner similar to that which the referenced patent uses for drum type boilers by providing a novel signal representing the heat released to the furnace of the boiler.